The muffle kiln is shown in Fig. 46.
A is the muffle-box or chamber for the pottery.
B B B B B B B are the arches supporting the box and other parts of the kiln.
C C The doors where the fuel and pottery respectively are put in.
D D D The passages for the heated air.
E The ash-pan.
This fires at a low degree of heat, compared with the open-fire kiln. It is mostly used for under-glaze pottery and for glazing. In this kiln the fire could not come in direct contact with the ware, so that saggers are unnecessary; instead, there is a system of shelves upon which the pottery is placed. The fuel, which may be wood, is fed through a door on one side of the lower part of the kiln, with an ash-box below. A brick arch supports the muffle-box or chamber where the pottery is placed. On the opposite side from the furnace door is the opening through which the pieces are put into the kiln. This is carefully bricked up before the fire is started. The hot air passes through a passage extending around all sides and above the box, and the outlet is over the door where the pottery goes in. A slab extending nearly across this outlet controls the draught.